The present invention relates to heap leach mining. More specifically, the present invention relates to a method and apparatus used in heap leach mining that involves solar heating and distribution of a leach solution prior to its percolation through an ore heap.
Copper is frequently recovered from extremely low grade ores and from mine wastes by a process known as heap leaching. The term xe2x80x9cheapxe2x80x9d as used in the art means a bed of low grade ore that has been suitably spread over a prepared surface or xe2x80x9cpadxe2x80x9d. A dilute sulfuric acid solution is percolated through the heap and the xe2x80x9cpregnantxe2x80x9d copper-bearing acid solution is collected after it has percolated through the heap. Typically, the pad on which the heap rests is formed from an impervious material, such as sheets of plastic film (polyethylene), asphalt and/or compacted clay. The heap is often times constructed on an incline such that the copper-bearing leach solution that has percolated through the heap is collected in a reservoir.
The heap of ore is normally piled onto each impervious pad to a depth of 10 to 30 feet after the ore has been pre-crushed to a sufficiently small size to enable the leach solution to reach the metallic-mineral particles contained in the ore. It is a characteristic of heap leach mining that the maximum amount of copper can be recovered from the ore over a long period of time, typically measured in months and years. For example, a typical heap leach operation may reach a maximum copper recovery in a period of two to three years.
After the leach solution has passed through the ore heap, the copper-bearing leach solution is subjected to a solvent extraction process in which the copper is transformed from dilute leach solution into a pure and concentrated solution termed advance electrolyte. After the extraction, the advance electrolyte is subjected to an electro-winning process in which the dissolved copper plates out onto permanent stainless steel plates or pure thin copper sheets used as cathodes. The plated copper on each of the stainless steel cathodes can be removed and processed. The copper sheets, when used, grow in weight and can be processed as required.
Conventional percolation systems for heap leach mining normally includes a series of individually spaced emitter tubes extending over the heap of ore. Each of the spaced tubes receives the leach solution and includes a series of openings to permit the leach solution to be distributed for uniform percolation down through the heap. An example of such a system is shown in the Krauth U.S. Pat. No. 5,030,279. In this type of system, each of the individual tubes are laid out over the ore heap in the desired spaced relationship such that the leach solution adequately permeates through the ore heap.
In heap leach operations, it has been found that metal recoveries are considerably lower in the winter season due to lower temperatures of the ore and the leach solution. Heap leach operations, utilizing both chemical and biological leach solutions, tend to operate most efficiently at temperatures above ambient. While others have proposed heating the leach solution at a location remote from the heap, heating the leach solution at such a remote location has proven to be very inefficient due to the substantial amount of heat lost to atmosphere as the leach solution is distributed to the tubing placed on the heap.
An example of a type of heaping and distributing system proposed to solve such problems is shown in the Lane U.S. Pat. No. 6,149,711. In the Lane ""711 patent, a distribution mat having a series of individual emitter tubes connected to each other by heat absorbing panels is disclosed. The distribution mat is laid out over the heap and the temperature of the leach solution passing through the emitter tubes is elevated above ambient prior to the leach solution circulating down through the ore heap through openings formed in the emitter tubes of the mat. Although this type of system has proven to be an effective way to increase the temperature of the leach solution prior to the distribution over the ore heap, the pores formed in the emitter tubes create a problem in forming the mat and substantially increase the cost of the mat.
Therefore, it is an object of the present invention to provide a method of increasing the rate at which a semi-precious metal can be extracted from an ore heap. It is a further object of the invention to provide a method that solar heats a leach solution prior to the leach solution being distributed over the bed of ore. It is an additional object of the invention to provide a method that solar heats the leach solution immediately prior to the leach solution being percolated through the ore heap. Further, it is an object of the invention to provide an apparatus that increases the temperature of the leach solution immediately prior to the leach solution being applied to the ore heap. It is a further object of the invention to provide an apparatus that solar heats the leach solution prior to its percolation through the ore heap.
The present invention is a method and apparatus for heating a leach solution that is applied over the surface of the dump. The apparatus is a heating tube and mat assembly having a plurality of tubes joined to a underlying mat.
Each of the heat absorbing tubes is joined to the heat absorbing/heat retaining mat providing for a generally flat cover for the surface of the leach dump. The mat and tubes positioned over the surface of the dump collect solar energy. The solar energy absorbed by each of the heat absorbing tubes is transferred by conduction to the leach solutions passing through the heat absorbing tubes. The transferred heat elevates the temperature of the leach solution above ambient prior to the leach solutions being supplied to a separate array of drip lines or emitter tubes positioned on the surface of the ore heap.
In the preferred embodiment of the invention, the heat absorbing tubes are joined by a flexible material such that the heat absorbing tubes absorb solar energy that is then transferred to the leach solution. Preferably, the material used to form the heat absorbing mat is sufficiently flexible to allow the distribution mat to be rolled onto a spool when not in use. The spacing between the individual heat absorbing tubes is determined by the specific application for the distribution mats.
Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings.